1. Field of the Invention
The present invention relates generally to surge protector circuits that protect electrical equipment connected to electrical lines, and relates more particularly to an alternating current (AC) building entrance surge protector circuit that uses a combination of metal-oxide varistors and gas-discharge tubes to get improved performance.
2. Description of the Relevant Art
Electrical lines, such as AC power lines, are subject to power surges of an abnormally high current and/or voltage that may be caused by lightning or short circuits. A power surge can cause permanent damage to electrical and electronic devices connected to the power line. Surge protectors have been developed to detect surges and to block the surge before it reaches devices on the power line. Roughly speaking, surge protectors are divided into two different typesxe2x80x94permanently-wired and plug-connected.
Building-entrance surge protectors are permanently wired to power lines where they enter a building and are designed with a relatively large surge capacity. They are located where power lines enter a building so that they can protect the portions of the power lines that run inside the building as well as devices which are connected to the power lines. Another feature of building-entrance surge protectors is that, because of their location, they can be grounded directly to building ground wires, which have good connection to the Earth itself, providing excellent grounding for large surges, and, therefore, improved protection.
Plug-connected surge protectors, on the other hand, are generally located on the inside of a building, xe2x80x9cdownstreamxe2x80x9d from any building-entrance surge protector. For instance, it is common to place a plug-connected surge protector on an AC power line between a wall outlet and a computer. Because they are generally located indoors, plug-connected surge protectors are not generally required to be as rugged as building-entrance surge protectors, and typically have a lower surge current capacity.
Two factors that are important in the design and performance of a protector circuit are the turn-on voltage and limiting voltage. The turn-on voltage is the voltage at which the protector begins shunting a surge to ground, i.e., the xe2x80x9cinputxe2x80x9d voltage that triggers the operation of the surge protector. The limiting voltage (also called xe2x80x9clet-throughxe2x80x9d voltage) is the voltage that is passed through to the power line when the protector is shunting a surge to ground, i.e., the xe2x80x9coutputxe2x80x9d voltage that devices downstream of the protector may experience in the event of a surge.
AC power protector circuits have an inherent design compromise: If the turn-on voltage is set low, the protector circuit will be activated by even minor over-voltages, and may wear out quickly or interfere with the power supplied to the protected devices. On the other hand, if the turn-on voltage is set high, then the limiting voltage under surge conditions will tend to be high, which is undesirable because it may damage the protected equipment.
A standard design for building-entrance AC power protectors uses metal-oxide varistors (MOVs) with a turn-on voltage of 230V. This type of protector circuit, however, has the disadvantage that the capacitance of the MOVs interferes with Power Line Carrier (PLC) communication systems that send and receive high frequency communication signals over the power lines within a building. The MOVs attenuate the high-frequency signals, thus interfering with the operation of the system. Also, this type of protector circuit typically uses fuses for the MOVs in order to pass standard safety tests.
Combinations of MOVs and gas-discharge tubes (GDTs) have been used commercially for plug-connected surge protectors that are connected by line-cord and AC plug to the building wiring system. However, protectors of this design have apparently not been used for permanently-wired building entrance AC surge protectors.
In summary, the present invention is a building-entrance surge protector for single-phase or multi-phase power lines that has a basic protector sub-circuit of a metal-oxide varistor (MOV) and a gas-discharge tube (GDT) connected in series between the power line and building ground at a location adjacent to the building entrance.
In its most fundamental form, the present invention includes (1) a metal-oxide varistor having two electrodes and being coupled at one electrode to a power line, and (2) a gas-discharge tube having a line electrode connected to the other electrode of the metal-oxide varistor and having a ground electrode connected to the building ground. Preferably, the gas-discharge tube has two line electrodes and there is a second metal-oxide varistor connected in parallel to the first metal-oxide varistor between the second gas-discharge tube electrode and the power line.
In a more complex form, the present invention includes one or more protector sub-circuits connected between each power line of multi-phase power lines, wherein each protector sub-circuit includes two metal-oxide varistors each having two electrodes and each being coupled at one electrode to a power line, and a gas-discharge tube having two line electrodes connected to the other electrodes of the metal-oxide varistors and having a ground electrode connected to the building ground at the location adjacent to the building entrance. Optionally, there is also a coupling capacitor connected between the power lines.
Basically, the present invention uses metal-oxide varistors (MOVS) and gas-discharge tubes (GDTs) in a permanently-wired, building entrance surge protector circuit intended for fixed installation across an AC service with a permanent, short connection to the building ground. The present invention eliminates series-connected thermal fuses that prior protector circuits have used to limit currents to pass standard safety tests, with consequent savings of cost and space and increased protector ratings, since the full surge absorbing capabilities of the MOVs and GDTs are utilized.
The present invention has improved turn-on voltage and surge limiting voltage, which increase the expected service life of the protector. The protector also survives, without damage, fault conditions in which AC voltage of 240VAC is applied across (normally) 120V terminals. Also, the presence of the GDTs makes it possible to pass standard safety tests without the addition of special thermal fusing circuits, and without damaging any circuit elements.
One optional feature of the present invention is the use of a three-element GDT (two line electrodes and a ground electrode) shared by two MOVs. This reduces cost and protector size, and improves the equalization of current in the various parts of the protector.
The gas-discharge tubes of the present invention also isolate the capacitance of the metal-oxide varistors from the power lines during normal (non-surge) operation, which is a significant benefit to the use of Power Line Carrier (PLC) communications because the MOVs, unlike those of prior protectors, do not attenuate the PLC signals carried in the power lines during normal operation. Optionally, the present invention can include a coupling capacitor so that PLC signals being transmitted in one power line of a multi-phase connection will be coupled to the other power line and to PLC receivers on the other power line.
The features and advantages described in the specification are not all inclusive, and particularly, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification and claims hereof. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter, resort to the claims being necessary to determine such inventive subject matter.